The present invention relates to phacoemulsification surgery and more particularly to a thermal management algorithm in which the amplitude of power applied to a phacoemulsification hand piece is varied in proportion to temperature.
The human eye functions to provide vision by transmitting light through a clear outer portion called the cornea, and focusing the image by way of a crystalline lens onto a retina. The quality of the focused image depends on many factors including the size and shape of the eye, and the transparency of the cornea and the lens. When age or disease causes the lens to become less transparent, vision deteriorates because of the diminished light which can be transmitted to the retina. This deficiency in the lens of the eye is medically known as a cataract. An accepted treatment for this condition is surgical removal of the lens and replacement of the lens function by an artificial intraocular lens (IOL).
In the United States, the majority of cataractous lenses are removed by a surgical technique called phacoemulsification. A typical surgical hand piece suitable for phacoemulsification procedures consists of an ultrasonically driven phacoemulsification hand piece, an attached hollow cutting needle surrounded by an irrigating sleeve, and an electronic control console. The hand piece assembly is attached to the control console by an electric cable and flexible tubing. Through the electric cable, the console varies the power level transmitted by the hand piece to the attached cutting needle. The flexible tubing supplies irrigation fluid to the surgical site and draws aspiration fluid from the eye through the hand piece assembly.
The operative part in a typical hand piece is a centrally located, hollow resonating bar or horn directly attached to a set of piezoelectric crystals. The crystals supply the required ultrasonic vibration needed to drive both the horn and the attached cutting needle during phacoemulsification, and are controlled by the console. The crystal/horn assembly is suspended within the hollow body or shell of the hand piece by flexible mountings. The hand piece body terminates in a reduced diameter portion or nosecone at the body's distal end. Typically, the nosecone is externally threaded to accept the hollow irrigation sleeve, which surrounds most of the length of the cutting needle. Likewise, the horn bore is internally threaded at its distal end to receive the external threads of the cutting tip. The irrigation sleeve also has an internally threaded bore that is screwed onto the external threads of the nosecone. The cutting needle is adjusted so that its tip projects only a predetermined amount past the open end of the irrigating sleeve.
During the phacoemulsification procedure, the tip of the cutting needle and the end of the irrigation sleeve are inserted into the anterior capsule of the eye through a small incision in the outer tissue of the eye. The surgeon brings the tip of the cutting needle into contact with the lens of the eye, so that the vibrating tip fragments the lens. The resulting fragments are aspirated out of the eye through the interior bore of the cutting needle, along with irrigation solution provided to the eye during the procedure, and into a waste reservoir.
Throughout the procedure, irrigating fluid is pumped into the eye, passing between the irrigation sleeve and the cutting needle and exiting into the eye at the tip of the irrigation sleeve and/or from one or more ports, or openings, cut into the irrigation sleeve near its end. The irrigating fluid protects the eye tissues from the heat generated by the vibrating of the ultrasonic cutting needle. Furthermore, the irrigating fluid suspends the fragments of the emulsified lens for aspiration from the eye.
Power is applied to the hand piece to vibrate the cutting needle. In general, the amplitude of needle movement (or vibration) is proportional to the power applied. In conventional phacoemulsification systems, the needle vibrates back and forth producing a longitudinal needle stroke. In improved systems, the needle may be caused to vibrate in a twisting or torsional motion. Regardless of the type of vibration, the magnitude of vibration (or amplitude of needle stroke) varies with applied power.
One complication that may arise during the procedure is burning of the cornea at the incision site. These corneal burns are caused by heating of the needle (and surrounding sleeve) at the corneal incision. The inventors have found that this heating is dependent on three basic factors: the amount of power applied to the hand piece (which in turn determines the magnitude of needle vibration or amplitude of needle stroke); the amount of fluid flow through the eye (since the fluid carries heat away); and the amount of friction between the needle and the surrounding sleeve at the incision (as can be appreciated, the tighter the fit between the sleeve and the needle, the more friction, and the more heat produced as the needle vibrates).
In other words, heat is produced at the corneal incision as the cutting needle rubs against the surrounding irrigation sleeve. This heat is normally dissipated by fluid flowing through irrigation sleeve, into the anterior chamber of the eye, and out of the eye through the aspiration lumen. The friction between the cutting needle and the sleeve at the corneal incision site can vary depending on the characteristics of the incision. Generally, a smaller incision (which is desirable from a surgical perspective) can lead to a greater friction force between the needle and the sleeve as the walls of the incision press the sleeve against the needle. In such a case, when the needle is vibrated, heat is produced. If the fluid flowing through the eye is insufficient (or if too much heat is produced), a corneal burn can result. Corneal burns are problematic because they distort the cornea resulting in distorted vision. Since cataract surgery has gravitated toward smaller and smaller incisions, the risk of corneal burns appears to be increasing.